Failsafe control of power transmission and torque is a continuing problem in many fields of application. An application of significant importance to automotive chassis controls is the operation of power steering systems in motor vehicles, where the power steering assist is an integral part of modern driving equipment. Power steering is highly useful when making sharp turns at slow speeds or in parking an automobile. In such situations, a steering effort by the driver must be matched with a greater effort by the power steering mechanism. Too-little assist effort from the power steering, requiring strenuous effort by the driver to maneuver or park, is unsatisfactory.
An opposite situation is encountered in high-speed freeway driving, where a driver is able to steer manually with virtually no assistance from the power steering assist mechanism. The power steering mechanism must “know” that greater effort is required in the parking situation, in order for the power steering assist mechanism to assist the driver. In the second situation, however, any excessive assist the power steering mechanism would not be helpful, since the driver can easily maneuver and control the vehicle without assistance. Of course, the assistance is in the direction of the driver's effort so that the assist mechanism is not fighting the driver.
One problem with power steering systems is controlling the amount and direction of output by the assist mechanism in response to the driver steering input. Driver steering input is relatively easy to measure, using a torsion bar across input and output steering shafts and a Hall effect sensor in the steering column of an automobile or truck, as disclosed in U.S. Pat. No. 6,107,767. Power output by the assist mechanism, however, has been difficult to quantify in a cost-effective way. In U.S. Pat. Appl. Publ. US2001/0016793, published Aug. 23, 2001, for instance, there is no attempt to measure assist mechanism torque directly, but rather the steering correction depends on a series of calculations involving vehicle speed, deceleration, and increases or decreases in driver torque effort. U.S. Pat. No. 5,257,828 uses motor speed and yaw rate to control steering assistance, rather than trying to measure torque directly. U.S. Pat. No. 5,504,403 uses a driver torque signal and a series of closed-loop feedback calculations involving gain control to minimize feedback times and eliminate sluggishness in the steering assist. There is no attempt to measure and thus directly control the output torque of the steering assist mechanism. Thus, it may require a greater amount of time for the assist mechanism to respond to the driver.
What is needed is a better system and method for monitoring the effective output of a power steering assist mechanism in relation to the steering effort by the vehicle operator, to make sure that the assist is within reasonable bounds for a desirable level of steering assist and that it if it exceeds these bounds due to a system malfunction, it brings the system to a safe, limited function operating state.